This invention relates to a switched-mode power supply circuit for converting a DC input voltage into a DC output voltage, comprising a series arrangement of a controllable power switch and an inductive element coupled between the input voltage terminals, control means for rendering the power switch alternately conducting and non-conducting, and a rectifier coupled to the inductive element for making the output voltage available, the inductive element and a capacitor coupled thereto forming a part of a resonant circuit in the periods of time when the switch and the rectifier are currentless, in which resonant circuit a voltage oscillation is then present.
Such switched-mode power supply circuits including a resonant circuit are known. Examples are described in Netherlands patent application No. 8502339, which corresponds to U.S. Pat. No. 4,694,385 (9/15/87). The output voltage can be maintained substantially constant in spite of possible variations of the input voltage and/or of a load connected to the output voltage, for example, because the frequency of the control signals applied to the controllable switch is controlled as a function of this output voltage. The control signals may originate from an oscillator, or, in a different manner, the circuit may be self-oscillating.